Expose backend session limits through prometheus stats.

[fancycode]

Resolves #787

@pmarini-nc FYI, don't know if you have the possibility to test this from source

[pmarini-nc]

@fancycode , just tested it and it works fine. Thanks a lot!
Only thing maybe it is worth adding the global limit as well?

[fancycode]

Only thing maybe it is worth adding the global limit as well?

Not sure what you mean with "global limit". If the signaling server is running with compatibility settings (i.e. not with explicit backend definitions), the session limit will be available in the compat backend through the stats.

[pmarini-nc]

Hello,

My understanding until now was the following: if a backend defines it, it gets its value of sessionlimit. If not it gets the value of a global sessionlimit, as defined in section [backend].

However this seems not the case, that is: there is no fallback sessionlimit (like there is one for secret). If so, when we don't specify a sessionlimit in the backend, there is no limit, correct?

[fancycode]

However this seems not the case, that is: there is no fallback sessionlimit (like there is one for secret). If so, when we don't specify a sessionlimit in the backend, there is no limit, correct?

Right, adding a global sessionlimit as fallback for backends that have none defined is tracked in #796